AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), 2024: Question 4

✅ Correct answer

Potable water

💡 Key knowledge

• Potable water = water that is safe to drink.
• Ground water is water found underground.
• Waste water is dirty used water.

❌ Common errors

• Choosing ground water because it sounds like “natural” water.
• Choosing waste water because it may be treated, but it is not the name for safe drinking water.

✅ Correct answers

Ozone
Ultraviolet light

💡 Key knowledge

• Water can be sterilised by killing microorganisms.
• Ozone is used as a disinfectant.
• Ultraviolet light kills microorganisms too.

🧠 Exam technique

• Only tick the methods that kill microbes.
• Read every option carefully: some are treatment processes, but not sterilisation.

❌ Common errors

• Ticking filtration because it removes particles, but it does not sterilise the water.
• Ticking carbon dioxide or electrolysis, which are not used to sterilise drinking water.

✅ Correct answer

Measuring cylinder

💡 Key knowledge

• A measuring cylinder is used to measure liquid volume accurately enough for school experiments.
• A beaker is less accurate.
• A conical flask is mainly for mixing, not measuring volume.

🧠 Exam technique

• Choose the equipment that gives the best measurement, not just something that can hold the liquid.
• For 100 cm³, a measuring cylinder is the expected GCSE answer.

❌ Common errors

• Choosing a beaker because it can contain liquid.
• Choosing a conical flask because it has volume markings, but it is not the most suitable here.

📐 Calculation

Use: mass of dissolved solids = mass of evaporating basin and solids − mass of evaporating basin

1. From the table, sea water:
   mass of evaporating basin and solids = 114.0 g
   mass of evaporating basin = 110.5 g
2. Subtract:
   114.0 − 110.5 = 3.5
3. Write the answer with units:
   X = 3.5 g

✅ Correct answer

3.5 g

🧠 Exam technique

• Always subtract the empty evaporating basin from the basin + solids.
• Keep the decimal place aligned.
• Use the correct unit: g.

❌ Common errors

• Doing the subtraction the wrong way round.
• Giving 0.5 g or 4.5 g from an arithmetic slip.
• Forgetting units.

✅ Correct matches

• Dependent variable → Mass of dissolved solids
• Control variable → Volume of water

💡 Key knowledge

• Dependent variable = the thing you measure.
• Control variable = the thing you keep the same.
• In this investigation, the mass of dissolved solids changes depending on the type of water.

🧠 Exam technique

• Ask: What is being measured? → dependent variable.
• Ask: What is kept the same? → control variable.
• The type of water is the independent variable, even though it is not one of the boxes on the left.

❌ Common errors

• Mixing up dependent and control variables.
• Matching mass of evaporating basin to a variable: this is measured, but it is not the key variable in the investigation.
• Matching room temperature incorrectly when the method does not focus on changing temperature.

✅ Correct answers

• Complete the y-axis scale at 2.0, 2.5, 3.0
• Plot and label the magnesium ion bar at 1.3 g
• Plot and label the sulfate ion bar at 2.7 g

💡 Key knowledge

• Bar charts need a sensible, even scale on the y-axis.
• Each bar height must match the data exactly: calcium 0.4 g, magnesium 1.3 g, sulfate 2.7 g.
• For this graph, the existing calcium bar is already shown.

🧠 Exam technique

• Use equal intervals on the y-axis.
• Make sure the bars are the correct height and are clearly labelled.
• If the bar is plotted correctly but the label is missing, you may lose the labelling mark.

❌ Common errors

• Leaving the y-axis scale incomplete.
• Plotting magnesium or sulfate to the wrong height.
• Drawing bars with gaps or unequal widths that do not look like a proper bar chart.
• Forgetting to label the bars, or labelling the wrong bar.

🧠 What top responses did well

• Used the exact science term: potable water.
• Selected the two correct sterilising methods only.
• Used the table data carefully for the calculation.
• Identified variables by role, not by guesswork.
• Plotted the graph neatly and matched the data precisely.

❌ Examiner-style reminders

• Read the command word: tick, calculate, identify, complete.
• In graph questions, check both the scale and the labels.
• For calculations, always include the unit.